1. Field of the Invention
The present invention relates to a CDMA (Code Division Multiple Access) mobile communication system having base stations connected to a communication network and mobile stations for carrying out communications with the base stations in the CDMA scheme, where each base station has a plurality of sectors dividing the cell of each base station. More specifically, the present invention relates to a spread code utilization scheme, a sector selection scheme, and a transmission power control scheme for this type of CDMA mobile communication system with sector configuration.
2. Description of the Background Art
The CDMA mobile communication system is a system which uses the CDMA scheme, which is one of the spectrum spread scheme, as a radio access scheme between the base stations and the mobile stations.
In the CDMA scheme, a plurality of channels (mobile stations) uses the identical frequency, so that transmission data sequences of different channels are multiplied by different spread codes having low correlations with each other and then multiplexed, so as to make each channel identifiable. In multiplying the spread code, the spread code with a rate (chip rate) faster than the data rate of the data sequence to be spread is used (usually, the PN sequence with a rate faster than the user data by several hundreds to several thousands times is used). By spreading with the spread code of higher chip rate, the transmission bandwidth can be made wider. When the transmission bandwidth becomes wider, the tolerable interference level becomes higher and a number of channels that can carry out communications simultaneously can be increased. At the receiving side, only the desired signals are put back into original waveforms by carrying out the despreading for multiplying the same spread code to the received data sequence. FIG. 26 shows an exemplary change of the waveform (frequency region) in the spectrum spread scheme.
In the CDMA scheme, the other channels cause the identical frequency interference, so that there is the so called Near/Far problem in which a channel transmitted by a mobile station near the base station causes a very large interference with respect to a channel transmitted by another mobile station far from the base station. Consequently, in order to carry out communications for more channels by using a limited bandwidth, that is, in order to increase the capacity, there is a need for a high precision transmission power control which can satisfy the required quality and make the transmission power small.
On the other hand, the soft handover is an important technique in the CDMA scheme. The soft handover is a technique for simultaneously connecting radio channels between a mobile station and a plurality of base stations (sites) when the mobile station moves from one base station to another. By connecting a plurality of radio channels simultaneously, the site diversity composition gain can be obtained so that the transmission power can be lowered and the interference can be reduced.
The sector configuration of a cell is also effective in increasing the capacity. lyhen the sector configuration is used, the interference can be reduced so that the capacity can be increased as much. In a case of the FDMA scheme, the sector configuration has been utilized in order to increase the capacity by shortening the frequency reuse distance, but in a case of the CDMA scheme, the capacity can be increased as much as the interference is reduced by the use of the sector configuration ALso, in a case of using the site diversity among sectors, it is easier to realize the maximal ratio combining compared with a case of using the site diversity among cells, so that the higher composition gain can be obtained. For these reasons, it can be said that the sector configuration is more effective for the CDMA scheme than the FDMA scheme.
Conventionally, the cell/sector judgement for the handover operation has been carried out at the mobile station side in order to reduce the control load on the base station side. In a case of using the CDMA scheme, the mobile station makes the cell/sector judgement by using codes because the neighboring cells/sectors use the identical frequency. Namely, the mobile station scans the spread codes which are specified in advance as the perch channels of the cells/sectors, and makes the cell/sector judgement by comparing the receiving levels of the perch channels after the despreading. From a point of view of shortening the scan time of the mobile station required at a time of turning the power on, it is preferable to use a smaller number of perch channel spread codes, so that the same set of perch channel spread codes are usually arranged repeatedly for reuse.
However, in the conventional CDMA mobile communication system, different perch channel spread codes are assigned to different sectors, so that a required number of perch channel spread codes increases when a number of sectors is increased, and there has been a problem that the correlation characteristic of the perch channel spread codes becomes poor when a number of codes with a good correlation characteristic is limited.
In addition, there has been a problem that the scan time at the mobile station becomes longer when a number of perch channel spread codes is increased.
Moreover, in the conventional CDMA mobile communication system, there has been a problem in that, when a number of sectors is increased, a frequency of the handovers among the sectors also increases, so that the exchange of control signals between the mobile station and the base stations is more frequently required and the control signal traffic is jammed.
Furthermore, in the conventional CDMA mobile communication system, a number of sectors used has been about three, but in conjunction with an increase of users it is expected that a number of sectors used in a future CDMA mobile communication system will be increased to about twelve or more, so that the above noted problems related to the increase of a number of sectors are expected to become more serious.